1. Field of the Invention
This invention relates to a method of reducing transient voltages in switched reluctance drive systems.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, xe2x80x9cThe characteristics, design and application of switched reluctance motors and drivesxe2x80x9d by Stephenson and Blake, PCIM""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector 15 may take many forms, including that of a software algorithm. Its output may also be used to generate a speed feedback signal. Current feedback is provided by a current transducer 17 for the or each phase winding. As discussed in the Stephenson paper cited above, reluctance machines can be operated in either a motoring or a generating mode. The input demand 18 can be torque or speed demand for motoring or a current or voltage demand for generating.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the xe2x80x9cDC linkxe2x80x9d of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the xe2x80x9cDC link capacitorxe2x80x9d, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called xe2x80x9cripple currentxe2x80x9d) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter. The cost and/or size of this capacitor is important in installations which are sensitive to drive cost and/or the space occupied by the drive, for example in aerospace or automotive applications.
The switched reluctance drive is essentially a variable speed system and is characterized by voltages and currents in the phase windings of the machine which are quite different from those found in traditional types of machines fed with an alternating current. As is well known, there are two basic modes of operation of switched reluctance systems: single-pulse mode and chopping mode, both of which are described in the Stephenson paper cited above. These are briefly described here as follows.
At a predetermined rotor angle, voltage is applied to the phase winding by switching on the switches in the power converter 13 and applying constant voltage for a given angle xcex8c, the conduction angle. When xcex8c, has been traversed, the switches are opened and the action of energy return diodes places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then typically a period of zero current until the cycle is repeated. It will be clear that the phase is drawing energy from the supply during xcex8c and returning a smaller amount to the supply thereafter. This is shown in FIG. 3(a). This mode is generally known as the single-pulse mode because there is only one pulse of voltage applied to the phase in a phase period. Single-pulse mode is normally used for the medium and high speeds in the speed range of a typical drive. Instead of opening both switches simultaneously, there are circumstances in which it is advantageous to open the second switch an angle xcex8f later than the first, allowing the current to circulate around the loop formed by the closed switch, the phase winding and a diode. This technique is known as xe2x80x9cfreewheelingxe2x80x9d and is used for various reasons, including peak current limitation and acoustic noise reduction. The inclusion of freewheeling is shown in FIG. 3(b). Freewheeling can be used over a broad speed range. The timing of the initiation of freewheeling and its period are speed dependent.
At zero and low speeds, however, the single-pulse mode is not suitable, due to the high peak currents which would be experienced, and the chopping mode is used, in which the peak current is limited to some predetermined value during the overall period of conduction. As for single-pulse control, there are two principal variants of the chopping mode. The simplest method is to open simultaneously the two switches associated with a phase winding, e.g. switches 21 and 22 in FIG. 2. This causes energy to be returned from the machine to the DC link and is sometimes known as xe2x80x9chard choppingxe2x80x9d. With any chopping scheme, there is a choice of strategy for determining the current levels to be used. Many such strategies are known in the art. One scheme uses a hysteresis controller which enables chopping between upper and lower current levels. A typical scheme is shown in FIG. 4(a) for hard chopping. At a chosen switch-on angle xcex8on (which is often the position at which the phase has minimum inductance, but may be some other position), the voltage is applied to the phase winding and the phase current is allowed to rise until it reaches the upper hysteresis current Iu. At this point both switches are opened and the current falls until it reaches the lower current I1 and the switches are closed again, repeating the chopping cycle. An alternative method is to open only one of the switches and allow freewheeling to occur and is known as xe2x80x9cfreewheel choppingxe2x80x9d or xe2x80x9csoft choppingxe2x80x9d. FIG. 4(b) shows the phase current waveform for a hysteresis controller using freewheeling or soft chopping.
U.S. Pat. No. 4,933,621 (MacMinn), incorporated herein by reference, proposes the use of freewheeling chopping to reduce the switching device losses by reducing the switching frequency, and to reduce the ripple current rating of the capacitor. U.S. Pat. No. 5,942,865 (Kim), incorporated herein by reference, describes a system for freewheeling at the end of a series of PWM pulses, the delay time being selected to reduce the radial forces on the stator and hence reduce the emitted acoustic noise. A similar approach is described in EP 700945 (Wu), incorporated herein by reference, where the freewheeling period is selected in relation to the resonant frequency of the stator, the intention being to actively cancel the vibration of the stator with a counteracting pulse of equal magnitude. Use of much longer periods of freewheeling has been proposed by WO 90/16111 (Hedlund) and U.S. Pat. No. 5,760,565 (Randall), both incorporated herein by reference, in order to reduce the peak flux and hence reduce the associated iron loss at high speeds.
None of the prior art discusses the use of freewheeling to reduce the voltage rating of the DC link capacitor. The cost of the capacitor is influenced by a number of requirements, e.g., operating temperature, life requirement, ripple current rating, internal impedance, etc., but one of the most important is the voltage rating. This rating is determined not by the nominal value of the DC link, but by the transient voltages appearing on that link and the requirement to have a safety margin above the highest expected transient. Particularly in low-voltage systems operating from 24V or 48V batteries, the cost of the capacitor is very dependent on the transient voltage specification.
According to embodiments of the invention, there is provided a method of reducing the magnitude of transient voltages in a switched reluctance drive system which comprises a reluctance machine having a stator with at least one phase winding and a moving part which is movable in relation to the stator, switch means connected across the or each phase winding which are configurable into an energizing mode in which the phase winding is energized through the switch means from a supply for a phase conduction period, a freewheeling mode in which there is no applied voltage and current in the winding recirculates, and a de-energizing mode in which the voltage across the at least one phase winding is reversed, and a DC link capacitor connected across the supply side of the switch means, the method comprising: initiating the energizing mode at the beginning of the phase conduction period of the at least one phase winding; initiating the freewheeling mode, causing a first transient voltage spike across the capacitor; and initiating the de-energizing mode a predetermined period after initiating the freewheeling mode, causing a second transient voltage spike across the capacitor.
It is generally conventional thinking that the switching times in a switched reluctance drive should be as short as possible. This is clearly particularly acute at high speeds. To introduce a freewheeling step to reduce the transient voltage is essentially in conflict with this understanding. The choice of predetermined time between initiating the freewheeling and the de-energizing modes is preferably traded off against the time taken for the overall switching operation. It will be appreciated that the time available for the switching operation decreases as machine speed increases. Up to now the opportunity to use a freewheeling step in order to ease the transient voltage burden on the DC link capacitor has not been recognized.
The magnitude of the second transient voltage spike will depend, in part, on the extent of decay of the voltage across the DC link capacitor. According to a preferred form of the invention, the point at which de-energizing mode is entered is chosen not to be above or to coincide with the point in the DC link voltage decay at which the addition of the second voltage spike will not exceed the first spike. This represents a trade-off between the magnitude of the spikes and the time between switching from freewheeling to de-energizing.
According to an embodiment of the invention, the predetermined period is fixed according to the decay of the first transient voltage spike.
According to a particular form of the invention there is provided a method of reducing the magnitude of transient voltages in a switched reluctance drive system which comprises a reluctance machine having a stator with at least one phase winding and a moving part which is movable in relation to the stator, switch means connected across the or each phase winding which are configurable into an energizing mode in which the phase winding is energized through the switch means from a supply for a phase conduction period, a freewheeling mode in which there is no applied voltage and the current in the winding recirculates as the voltage across the winding decays, and a de-energizing mode in which the voltage across the at least one phase winding is reversed, and a DC link capacitor connected across the supply side of the switch means, the method comprising: initiating the energizing mode at the beginning of the phase conduction period of the at least one phase winding; initiating the freewheeling mode, causing a first transient voltage spike across the capacitor; initiating the de-energizing mode after initiating the freewheeling mode, causing a second transient voltage spike across the capacitor, and adjusting the period between the first and second transient voltage spikes to balance the magnitude of the second transient voltage spike against the time taken to de-energize the winding.